Fiber-optic data links have recently emerged to meet telecommunication and computer technology bandwidth requirements. The trend towards more input/output (I/O) escapes per card edge, more total I/O in electronic packaging, and substrates which perform at high frequencies have created a need for dense fiber connections into electronic modules. Great difficulty has occurred in providing sufficient fiber connector I/Os for multi-chip modules to meet these needs. Thus, the invention deals with the need for more I/O connectors per multi-chip module.
In this specification, the term "module" includes several levels of packaging, as follows: A "substrate" the inner-most part of a module; in the preferred embodiment the substrate is primarily silicon or glass-ceramic. A "chip carrier" is a substrate having semiconductor chips placed thereon in a module, and the chip carrier is a higher level of packaging than the substrate. A "housing" is a frame around the chip carrier to seal or protect the chip carrier and is the outer-most part of a module. In the preferred embodiment described herein, the "module" encompasses a substrate, a chip carrier, and a housing, although at times the term module may be used to refer to one of these parts. A module may be refer to as either a single-chip module or multi-chip module (MCM) according to whether its contained chip carrier has single or multiple chips (i.e. a module may contain one or more chips). An example is the commercially-used thermal conduction module (TCM) constructed with alumina substrates, which is a form of MCM. An upper major surface of the TCM is covered with a thermal cooling structure, and the other major surface is covered with conductive I/O (input/output) pins which are used to plug the module into a computer framework. The substrate in a TCM is constructed with many internal layers of wiring to accommodate the interconnections among multiple chips on the upper substrate surface. The TCM has a thin, low profile shape to support internal cooling in the TCM. Direct contact heat sinks are used. The low profile chip carrier in the module having small edge surfaces compared to the top and bottom surfaces of the chip carrier. The module does not have sufficient area on any surface to provide a desired number of conventional pin-in-hole type connectors, and the narrow edges of the TCM do not contain any conductive I/O pins.
The number of I/Os per MCM is constrained by the inability to have connectors capable of being mounted on the narrow edges of an MCM. Current MCMs are being designed for use in thermal conduction modules (TCMs), which also make their upper surfaces unavailable for I/O connectors due to use for either piston or high conduction cooling fins to remove heat from the semiconductor chips mounted on the MCM surface. Pin array connectors, e.g. harcon, use the bottom of the MCM and prevent its use for optical fiber I/Os.
Optical fiber interconnection for computer modules offers a unique set of advantages in computer system architecture, package design, functionality, and performance. This invention resolves some of the problems associated with realizing these advantages, including simultaneous, precision alignment of optical fiber arrays which can be used with existing multichip module designs.
The preferred embodiment uses preferential crystallographic etching for making its V-grooves in silicon with photolithographic accuracy, which was published previously by Crow et al, "GaAs Laser Array Source Package", OPTICS LETTERS vol. 1, no. 1, p. 40-42 (July 1977). This work also established the feasibility of achieving the required fiber core alignment tolerances.
Brown et al U.S. Pat. No. 4,730,198 extends the V-groove fiber mounting techniques, regarding optimizing the alignment of optical sources (LED or laser) relative to a fiber optic silicon V-groove was treated by Balliet et al, IBM Technical Disclosure Bulletin vol. 24, no. 2, p. 1158-1160 (July 1981).
Methodology for preferred chemical etching of GaAs is given in Commerford and Zory, APPLIED PHYSICS LETTERS vol. 25, no. 4, p. 208-210 (Aug. 15, 1974) and Tsang and Wang, APPLIED PHYSICS LETTERS vol. 28, no. 1, p. 44-46 (January 1976). The extension of fiber V-groove structures to GaAs substrates with self-aligned monolithically integrated lasers was shown by Brady et al, in the IBM Technical Disclosure Bulletin vol. 26, no. 11, p. 5993-5995 (April 1994). Thus, it is clearly recognized that either silicon or GaAs V-grooving techniques may be economically used.
In U.S. Pat. No. 4,732,446 (Gipson et al) optical fibers were embedded in the body of a printed circuit board and interfaced with discrete chip carriers to create a simultaneous optical and electrical data bus network. Multiple printed circuit board layers, separated by aluminum heat sink plates, could be stacked and a card edge connector could form the interface to incoming data cables. Because of the multiplicity of chip carrier interfaces the modal noise and optical power losses of this structure are expected to significantly exceed those of the present invention.
U.S. Pat. No. 5,155,786 (Jacobowitz et. al.) describes an apparatus and method for interfacing external optical fibers into a fluid-sealed multichip module package, and provides for direct fiber attachment to optical sources or detectors within the module. The active optical components are attached to an optical submount, known as an optical sub-assembly, which is attached electrically to the substrate using "controlled collapse chip connection" process (called the C-4 (Controlled Collapsed Chip Connection) process). The C-4 (Controlled Collapsed Chip Connection) process is described on pages 30, 366, 1032, 1080, 1084 and 1135 in a book entitled "Microelectronics Packaging Handbook" by R. R. Tummala and E. J. Rymaszewski copyrighted in 1989 and published by Van Mostrand Reinhold.